FIG. 1 is a view of a small signal amplification circuit using a vacuum tube T1 as an amplification device wherein the vacuum tube T1 has been generally used for various audio systems. Here, the vacuum tube T1 can be substituted with a transistor.
As shown in FIG. 1, in the vacuum tube T1 of the amplification circuit, a self-bias unit 10 is connected in the vacuum tube T1 wherein a bias resistor R1 is connected with a grid in parallel and has a resistor R2 and a condenser C1 that are connected with a cathode in parallel. The self-bias unit 10 amplifies an AC input signal inputted into the grid and outputs to a plate.
The power unit 20 of the amplification circuit includes a smoothing unit 21 formed of a power transformer PT1, a rectifier T5, a resistor R4, and charging and discharging condensers C2 and C5 wherein the above elements are coupled in the shape of π. The 220V input AC power is converted into a DC power (B+) by the power transformer PT1. The DC power (B+) rectified and smoothed through the rectifier T5 and the smoothing unit 21 is supplied to the plate through the load resistor R3 connected in parallel with the plate of the vacuum tube T1.
The coupling condenser C4 of the amplification circuit is connected with the plate output terminal of the vacuum tube T1 for thereby disconnecting the DC component of the amplification output signal Vo1.
In the thusly constituted amplification circuit, when the DC power (B+) is supplied to the plate signal that is the positive terminal of the vacuum tube T1, the grid bias is generated at the resistor R2 of the self-bias unit 10 connected with the cathode that is the negative terminal of the vacuum tube T1 and the bias resistor R1 connected with the grid of the vacuum tube T1, respectively. Therefore, the vacuum tube T1 amplifies the AC input signal Vi1 inputted into the grid and outputs to the plate.
However, in the amplification circuit generally used in various audio systems, the effective resistance values are changed due to the capacity between both terminals of the bias resistor R1, so that the AC input signal Vi1 could be leaked through the bias resistor R1. As a result, the grid input signal of the vacuum tube T1 that is the amplification device could be attenuated.
In the case that the resistance value of the bias resistor R1 is set larger than over a few values of MΩ in order to prevent the leakage of the AC input signal Vi1 through the bias resistor R1, the grid bias voltage Vg occurring by the bias resistor R1 is sharply increased. Therefore, the grid input current of the vacuum tube T1 is increased and is excessively oscillated, so that the vacuum tube T1 is broken.
Therefore, the conventional amplification circuit should adapt the bias resistor R1 having a resistance value lower enough for thereby preventing the damage of the vacuum tube T1 that has been used as an amplification device. In this case, it is impossible to prevent the leakage of the AC input signal Vi1.
In the above amplification circuit, since the effective resistance value is changed due to the capacity between both ends of the load resistor R3 of the output terminal, the amplification output signal Vo1 may be leaked through the resistor R3. As a result, the plate output signal of the vacuum tube T1 that is the amplification device could be attenuated.
In particular, in the above amplification circuit, since the capacity of the coupling condenser C4 of the output terminal is set lower than the capacity of the charging and discharging condensers C2 and C5 included in the smoothing unit 21 of the power unit 20, the amplification output signal Vo1 is leaked through the charging and discharging condensers C2 and C5 of the smoothing unit 21. As a result, the plate output signal of the vacuum tube T1 that is the amplification device is attenuated.
In addition, in the above amplification circuit, the non-rectification AC component occurring due to the charging and discharging operations of the charging and discharging condensers C2 and C5 included in the smoothing unit 21 of the power unit 20 is mixed with the input and output signals Vi1 and Vo1, noises could be included in the input and output signals Vi1 and Vo1